Vehicular discharge lamp

ABSTRACT

A vehicular discharge lamp that emits light by a DC lighting system, the vehicular discharge lamp includes: an outer tube; a light emitting tube disposed in the outer tube and including a light emitting portion and fine tube portions connected to the light emitting portion; cathode-side and anode-side electrodes disposed in the light emitting tube; two lead wires connected to the electrodes; and a metal film or a metal oxide applied on an outer peripheral surface of the fine tube portion on an anode side of the light emitting tube, or the metal wire being wound on the outer peripheral surface. Inert gas having negative pressure is filled in a space outside the light emitting tube in the outer tube, and a negative high-voltage pulse is applied to the anode-side electrode at a time of start-up.

CROSS REFERENCE TO RELATED APPLICATION(S)

The present disclosure relates to the subject matters contained inJapanese Patent Application No. 2010-094085 filed on Apr. 15, 2010,which are incorporated herein by reference in its entirety.

FIELD

One aspect of the present invention relates to a vehicular dischargelamp. More particularly, one aspect of the invention relates to atechnical field where a size of a starting circuit or the like isreduced by reduction of a starting voltage that is achieved by applyinga negative high-voltage pulse to an anode-side electrode at the time ofa start-up of a vehicular discharge lamp.

BACKGROUND

Since a vehicle headlight requires the precise control of lightdistribution, unlike a general illumination lamp, the vehicle headlightneeds to be uniformly formed to have a rod shape and the light emittingform needs to have a high contrast ratio. Since a filament of anincandescent lamp or a halogen lamp has these characteristics, a halogenlamp or an incandescent lamp is widely used as a light source for avehicle headlight.

Meanwhile, the light intensity of a discharge lamp is higher than thatof an incandescent lamp or a halogen lamp. Accordingly, in a vehicleheadlight where a discharge lamp is used as a light source, it may bepossible to improve the brightness and to obtain a longer life of avehicle headlight using a discharge lamp is longer than that of avehicle headlight using each of an incandescent lamp and a halogen lamp.

Since the brightness of a discharge lamp is higher than that of anincandescent lamp or a halogen lamp and the life of a discharge lamp islonger than that of an incandescent lamp or a halogen lamp as describedabove, use of a discharge lamp as a vehicle headlight has spread inrecent years.

In general, a discharge lamp has a light emitting tube which holds apair of electrodes and which holds a gas, such as inert gas, is sealedin an outer tube that protects the light emitting tube or stabilizes thetemperature of the light emitting tube. The light emitting tube includesa light emitting portion in which discharge occurs and a pair of finetube portions that are provided on the opposite sides of the lightemitting portion. The light emitting portion is a portion where an arcis generated when discharge occurs, and the diameter of the lightemitting portion is larger than that of the fine tube portion.

When a high-voltage pulse is applied to an electrode, discharge occursin the light emitting portion of the light emitting tube, so that thedischarge lamp starts to light up.

As examples of a lighting system of the discharge lamp, there are bothAC lighting systems and DC lighting systems (see JP-A-2007-250225, forinstance). In the discharge lamp using a DC lighting system, a pair ofelectrodes are referred to as a cathode-side electrode and an anode-sideelectrode, respectively.

Since the polarities of the electrodes are not switched in the dischargelamp using a DC lighting system, the discharge lamp using a DC lightingsystem has an advantage of reducing the loads of the electrodes ascompared to a discharge lamp using an AC lighting system where acathode-side electrode and an anode-side electrode can be exclusivelydesigned for the loads of the electrodes, respectively.

Meanwhile, it can be important that a vehicular discharge lamp used as alight source for a vehicle light has reliable start-up (startingperformance of 100%) and a quick rising characteristic after thestart-up as compared to a general illumination discharge lamp.Accordingly, in the vehicular discharge lamp, in order to accelerate therising characteristic, the power immediately after lighting (starting)is increased and the pressure of the inert gas to be sealed isincreased.

Meanwhile, if the pressure of the inert gas is increased in order toaccelerate the rising characteristic, the starting voltage is increasedat the time of the start-up of the vehicular discharge lamp. For thisreason, the reliability of the start-up may be impaired.

Further, the size of a starting circuit which applies a starting voltageto the discharge lamp may be increased in order to increase the startingvoltage, leading to increased manufacturing costs.

SUMMARY

An object of exemplary embodiments of the present invention is toprovide a vehicular discharge lamp that improves the reliability of thestart-up of the vehicular discharge lamp and to reduce manufacturingcosts of the vehicular discharge lamp by the reduction of the startingvoltage.

With the above object in mind, there is provided a vehicular dischargelamp that lights up by a DC lighting system. The vehicular dischargelamp includes: an outer tube that is mounted on a socket; a lightemitting tube that is disposed in the outer tube, and that includes alight emitting portion and a pair of fine tube portions connected to thelight emitting portion on the opposite sides of the light emittingportion; a cathode-side electrode and an anode-side electrode that aredisposed in the light emitting tube; two lead wires which arerespectively connected to the cathode-side electrode and the anode-sideelectrode. Further at least a part of the cathode-side electrode and atleast a part of the anode-side electrode are disposed in the outer tube.Inert gas having negative pressure is filled in a space that is formedoutside the light emitting tube in the outer tube. A metal film or ametal oxide is applied, or a metal wire is wound, on the outerperipheral surface of the fine tube portion that is positioned on ananode side of the light emitting tube. A negative high-voltage pulse isapplied to the anode-side electrode at a time of start-up.

Accordingly, a negative high-voltage pulse is applied to the anode-sideelectrode, so that the vehicular discharge lamp starts up.

A vehicular discharge lamp according to an exemplary embodiment of thepresent invention lights up by a DC lighting system. The vehiculardischarge lamp includes: an outer tube that is mounted on a socket; alight emitting tube that is disposed in the outer tube, and thatincludes a light emitting portion and a pair of fine tube portionsconnected to the light emitting portion on the opposite sides of thelight emitting portion; a cathode-side electrode and an anode-sideelectrode that are disposed in the light emitting tube; two lead wireswhich are respectively connected to the cathode-side electrode and theanode-side electrode. Further at least a part of the cathode-sideelectrode and at least a part of the anode-side electrode are disposedin the outer tube. Inert gas having negative pressure is filled in aspace outside the light emitting tube in the outer tube. A metal film ora metal oxide is applied, or a metal wire is wound, on the outerperipheral surface of the fine tube portion that is positioned on theside of the light emitting tube corresponding to an anode. A negativehigh-voltage pulse is applied to the anode-side electrode at the time ofthe start-up.

Accordingly, it may be possible to reduce the starting voltage at thetime of the start-up of the vehicular discharge lamp, and to improve thereliability of the start-up of the vehicular discharge lamp and toreduce manufacturing costs of the vehicular discharge lamp by thereduction of the starting voltage.

In another embodiment of the present invention, tungsten to whichthorium oxide is added may be used as a material of the anode-sideelectrode.

Accordingly, losses caused by the discharge of electrons when ahigh-voltage pulse is applied to the anode-side electrode at the time ofthe start-up of the discharge lamp are reduced. Therefore, thetemperature rise of the anode-side electrode is suppressed and theerosion of the anode-side electrode is suppressed. As a result, it maybe possible to lengthen the life of the discharge lamp and to improvethe luminous efficiency of the discharge lamp.

In another embodiment of the present invention, the maximum innerdiameter of the light emitting portion may be set to 3 mm or less in adirection perpendicular to a direction where the pair of fine tubeportions are connected to each other.

Accordingly, since gaps between the inner surface of the light emittingportion and the anode-side electrode and the cathode-side electrode aresmall, it may be possible to reduce the starting voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

A general configuration that implements the various features ofexemplary embodiments of the invention will be described with referenceto the drawings. The drawings and the associated descriptions areprovided to illustrate embodiments of the invention and should not limitthe scope of the invention.

FIG. 1 is a view showing a vehicular discharge lamp according to anexemplary embodiment of the invention together with FIGS. 2 and 3, andis a schematic cross-sectional view of a vehicle headlight.

FIG. 2 is an enlarged side view of the discharge lamp of which a part isshown by a cross section.

FIG. 3 is an enlarged cross-sectional view showing a part of thedischarge lamp.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A vehicular discharge lamp according to an exemplary embodiment of theinvention will be described below with reference to the accompanyingdrawings. A vehicle headlight is provided with a vehicular dischargelamp.

The vehicle headlights 1 are disposed so as to be mounted on both theleft and right end portions of a front end portion of a vehicle body.

As shown in FIG. 1, the vehicle headlight 1 includes a lamp housing 2that has a recess opened toward the front side and a cover 3 that closesthe opening of the lamp housing 2, and a lamp outer case 4 is formed bythe lamp housing 2 and the cover 3. An internal space of the lamp outercase 4 is formed as a lamp chamber 5.

An insertion hole 2 a, which passes through the lamp housing 2 in afront-and-rear direction, is formed at a rear end portion of the lamphousing 2, and the insertion hole 2 a is closed by a back cover 6. Apositioning hole 2 b, which passes through the lamp housing 2 in anup-and-down direction, is formed at a lower end portion of the lamphousing 2.

In the lamp chamber 5, a reflector 7 is supported by an optical axisadjustment mechanism (not shown) so as to be tiltable. Amounting hole 7a, which passes through the reflector in the front-and-rear direction,is formed at a rear end portion of the reflector 7.

A discharge lamp (vehicular discharge lamp) 8 is horizontally mounted onthe mounting hole 7 a of the reflector 7.

A discharge lamp lighting device 9 is mounted on the positioning hole 2b of the lamp housing 2. The discharge lamp lighting device 9 includes alighting circuit (not shown) that is received in a case body 10. Aninput connector 11 is provided on the outer peripheral surface of thecase body 10, and an output connector 12 is provided on the uppersurface of the case body 10.

The input connector 11 is connected to a power supply circuit (notshown) by a connecting cord (not shown).

The output connector 12 is connected to a starting device 14 through apower supply cord 13, and a connector 14 a of the starting device 14 isconnected to a socket (to be described below) of the discharge lamp 8.

A power supply voltage of the power supply circuit is increased by alighting circuit of the discharge lamp lighting device 9 and ahigh-voltage pulse is applied to the discharge lamp 8 through the powersupply cord 13 and the starting device 14, so that discharge starts. Asa result, the discharge lamp 8 lights (starts) up. A DC lighting systemis used as a lighting system of the discharge lamp 8.

An extension 15, which shields a part of each component disposed in thelamp chamber 5, is provided in the lamp chamber 5. A shade (not shown),which blocks a part of the light emitted from the discharge lamp 8, isdisposed in the lamp chamber 5.

A main body 16 is connected to a socket 17, so that the discharge lamp 8is formed (see FIG. 2).

The main body 16 includes an outer tube 18 and a light emitting tube 19disposed in the outer tube 18.

The outer tube 18 includes a closing portion 18 a and a holding portion18 b that are integral with each other. The closing portion 18 a coversthe light emitting tube 19 and the like, and the holding portion 18 bprotrudes forward from a front end portion of the closing portion 18 a.

The light emitting tube 19 is made of a ceramic such as alumina, andincludes a light emitting portion 20 and fine tube portions 21 and 22that are connected to both front and rear ends of the light emittingportion 20, respectively. Each of the fine tube portions 21 and 22 isformed substantially in the shape of a cylinder extending in thefront-and-rear direction, and the outer diameter of each of the finetube portions 21 and 22 is smaller than that of the light emittingportion 20.

Iodide and inert gas such as xenon or argon are sealed in the lightemitting portion 20 and the end portions of the fine tube portions 21and 22 close to the light emitting portion 20. A middle portion of thelight emitting portion 20 in the front-and-rear direction is formed of astraight portion 20 a that is formed substantially in the shape of acylinder extending in the front-and-rear direction (see FIG. 3). Themaximum inner diameter of the light emitting portion 20, that is, aninner diameter D of the straight portion 20 a is set to, for example, 3mm or less and is preferably set in the range of 1 to 2 mm.

A cathode-side electrode 23 and an anode-side electrode 24, which areformed to extend in the front-and-rear direction, are disposed in thefine tube portions 21 and 22, respectively, so as to be separated fromeach other in the front-and-rear direction. A rear end portion 23 a ofthe cathode-side electrode 23 and a front end portion 24 a of theanode-side electrode 24 are positioned in the light emitting portion 20.

Meanwhile, an example where the cathode-side electrode 23 is positionedon the front side and the anode-side electrode 24 is positioned on therear side has been described above, but conversely, the cathode-sideelectrode 23 may be positioned on the rear side and the anode-sideelectrode 24 may be positioned on the front side.

The cathode-side electrode 23 is formed in the shape of, for example, aneedle having a small diameter and the anode-side electrode 24 is formedin the shape of, for example, a thick rod so that the cross-sectionalarea of the anode-side electrode 24 is larger than that of thecathode-side electrode 23. Further, tungsten to which thorium oxide isadded is an example of material used as the anode-side electrode 24.

A metal film 25 or a metal oxide film 25 is applied or a metal wire 25is wound on the outer peripheral surface of the fine tube portion 22that is positioned on the side of the light emitting tube 19corresponding to an anode.

A first lead wire 26 is connected to the front end of the cathode-sideelectrode 23 (see FIGS. 2 and 3). The first lead wire 26 includes aportion that protrudes forward from the front fine tube portion 21 ofthe light emitting tube 19, passes through the holding portion 18 b, andprotrudes toward the outside of the outer tube 18. The portion of thefirst lead wire 26, which protrudes toward the outside of the outer tube18, is bent in a predetermined direction, and is connected to a firstconnection terminal (not shown) of which the rear end portion isprovided in the socket 17.

The first lead wire 26 is formed of a first portion 26 a that isdisposed in the fine tube portion 21 and connected to the cathode-sideelectrode 23, and a second portion 26 b that is connected to the frontend of the first portion 26 a. The first portion 26 a is made of, forexample, molybdenum, and the second portion 26 b is made of, forexample, niobium. A portion of the second portion 26 b, which isdisposed in the fine tube portion 21, is sealed by frit glass (lowmelting point glass).

A part of the first lead wire 26 is held by the holding portion 18 b ofthe outer tube 18, and an insulating sleeve 27 is coated on a part ofthe portion of the first lead wire 26 that protrudes toward the outsideof the outer tube 18. The insulating sleeve 27 is made of an insulatingmaterial, such as glass or ceramic.

A second lead wire 28, which extends substantially in the front-and-reardirection, is connected to the rear end of the anode-side electrode 24.The second lead wire 28 protrudes rearward from the rear fine tubeportion 22 of the light emitting tube 19. The rear end portion of thesecond lead wire 28 is connected to a second connection terminal (notshown) that is provided in the socket 17.

The second lead wire 28 is formed of a first portion 28 a that isdisposed in the fine tube portion 22 and connected to the anode-sideelectrode 24, and a second portion 28 b that is connected to the rearend of the first portion 28 a. The first portion 28 a is made of, forexample, molybdenum, and the second portion 28 b is made of, forexample, niobium. A portion of the second portion 28 b, which isdisposed in the fine tube portion 22, is sealed by frit glass.

A getter 29, which is mounted on the second lead wire 28, is disposed inthe outer tube 18. The getter 29 has a function of preventing thedeterioration of the luminous efficiency of the discharge lamp 8 byadsorbing impurities (impure gas) that may exist in the outer tube 18.

In the discharge lamp 8, an inert gas such as argon gas or nitrogen gasis sealed as a shroud gas in a space that is formed outside the lightemitting tube 19 in the outer tube 18. The pressure of argon gas is setto 0.9 atm or less, and is preferably set to 0.1 atm or less.

A DC lighting system is used as a lighting system in the discharge lamp8 as described above, and a negative high-voltage pulse is applied tothe anode-side electrode 24, so that the discharge lamp 8 starts up.Accordingly, electrons are discharged from the anode-side electrode 24at the time of the start-up of the discharge lamp 8, and the dischargedelectrons collide with the cathode-side electrode 23. The anode-sideelectrode 24 functions as an electron generating member at the time ofthe start-up of the discharge lamp, and the cathode-side electrode 23functions as a ground member.

A high-voltage pulse is applied to the anode-side electrode 24 at thetime of the start-up of the discharge lamp 8 as described above, and ahigh-voltage pulse to be applied is negative. Accordingly, after thedischarge lamp starts (lights) up, electrons flow to the ground memberfrom the member to which a high-voltage pulse is applied, that is,electrons flow to the cathode-side electrode 23 from the anode-sideelectrode 24.

If inert gas having negative pressure is sealed in the space that isformed outside the light emitting tube 19 in the outer tube 18 when ahigh-voltage pulse is applied, dielectric barrier discharge occursbetween the second lead wire 28 and the metal film 25, the metal oxidefilm 25, or the metal wire 25 of the anode-side electrode to which thehigh-voltage pulse is applied.

When dielectric barrier discharge is caused, the inert gas around theanode-side electrode 24 to which a high-voltage pulse is applied isionized and electrons are generated. Accordingly, a strong electricfield is generated and ultraviolet light is also generated. Maindischarge occurs after the dielectric barrier discharge and electronsflow to the cathode-side electrode 23 from the anode-side electrode 24.However, since many electrons are excited around the anode-sideelectrode 24 by the strong electric field and ultraviolet lightgenerated due to the dielectric barrier discharge, the flow of electronsis rapidly increased with the excited electrons as “seeds”.

Since the flow of electrons is increased due to the effect of dielectricbarrier discharge as described above, the discharge lamp 8 reliablystarts up. Accordingly, it may be possible to reduce the startingvoltage. Further, since the starting voltage is reduced, it may bepossible to reduce the size of the starting circuit for applying thestarting voltage. As a result, it may be possible to reducemanufacturing costs.

Meanwhile, if a high-voltage pulse to be applied is positive, electronsflow to the anode-side electrode 24 from the cathode-side electrode 23.However, since a metal film 25 does not exist near the cathode-sideelectrode 23, the effect of dielectric barrier discharge is low and mostof the electrons do not exist. For this reason, it is difficult toreduce the starting voltage. Moreover, likewise, even when shroud gas isnot inert gas or the pressure of shroud gas is high, the effect ofdielectric barrier discharge is low. For this reason, it is difficult toreduce a starting voltage.

Further, since the heat transfer coefficient of argon gas is low amongthe heat transfer coefficients of inert gases, it may be possible toimprove the luminous efficiency of the light emitting tube 19 by a heatinsulating effect if an argon gas is sealed as a shroud gas in the spacethat is formed outside the light emitting tube 19 in the outer tube 18.

In the discharge lamp 8, as described above, the cathode-side electrode23 is formed in the shape of a needle having a small diameter and theanode-side electrode 24 is formed in the shape of a thick rod so thatthe cross-sectional area of the anode-side electrode 24 is larger thanthat of the cathode-side electrode 23. Since the anode-side electrode 24and the cathode-side electrode 23 are formed to have these shapes andsizes, electron decay is apt to occur on the side corresponding to ananode and streamers are easily generated by the concentration of anelectric field when a negative high-voltage pulse is applied to theanode-side electrode 24. Accordingly, it may be possible to reduce thestarting voltage to be applied to the anode-side electrode 24.

Further, since a negative high-voltage pulse is applied to theanode-side electrode 24 and the cathode-side electrode 23 is formed inthe shape of a needle having a small diameter, streamers are easilygenerated at the end of the cathode-side electrode 23 as describedabove, so that the end of the cathode-side electrode 23 is heated. Forthis reason, electrons are easily discharged from the end of thecathode-side electrode 23 and the transition of the discharge lamp fromthe time of the start-up to the time of stable lighting becomesreliable. As a result, it may also be possible to improve reliability inthe transition of the lighting of the discharge lamp.

Furthermore, tungsten to which thorium oxide is added is used as thematerial of the anode-side electrode 24 in the discharge lamp 8. Thoriumoxide is a metal material having a low work function. Accordingly, ifthorium oxide is added to tungsten, the work function of the anode-sideelectrode 24 is lowered. For example, the work function of tungsten is4.2 eV, but the work function of tungsten is reduced to 2.6 eV ifthorium oxide is added to tungsten.

Since thorium oxide having a low work function is added to tungsten asdescribed above, losses caused by the discharge of electrons when ahigh-voltage pulse is applied to the anode-side electrode 24 at the timeof the start-up of the discharge lamp are reduced. Accordingly, thestarting voltage is reduced, the temperature rise of the anode-sideelectrode 24 is suppressed, and the erosion of the anode-side electrode24 is suppressed. As a result, it may be possible to lengthen the lifeof the discharge lamp 8 and to improve the luminous efficiency of thedischarge lamp.

Further, since tungsten to which thorium oxide is added is used as thematerial of the anode-side electrode 24, it may be possible to suppressthe temperature rise of the anode-side electrode 24 at the time of thestart-up of the discharge lamp. Accordingly, it may be possible toreduce the size of the anode-side electrode 24 and to improve theluminous efficiency of the discharge lamp 8.

Since a high-voltage pulse is applied to the anode-side electrode 24 inthe discharge lamp 8, the applied high-voltage pulse reaches thecathode-side electrode 23 through the anode-side electrode 24 and theinner surface of the light emitting portion 20. Therefore, as gapsbetween the inner surface of the light emitting portion 20 and theanode-side electrode 24 and the cathode-side electrode 23 are reduced,it may be possible to reduce the starting voltage.

Accordingly, the inner diameter D of the straight portion 20 a, which isthe maximum inner diameter of the light emitting portion 20, is set to 3mm or less and is preferably set in the range of 1 to 2 mm in thedischarge lamp 8 as described above, so that gaps between the innersurface of the light emitting portion 20 and the anode-side electrode 24and the cathode-side electrode 23 are reduced. For this reason, astarting voltage is to be reduced.

Further, iodide and an inert gas such as xenon or argon have been sealedin the light emitting portion 20. However, if the inner diameter of thelight emitting portion 20 is reduced, the temperature of the lightemitting portion 20 rises at the time of the start-up of the dischargelamp. For this reason, more iodide is easily deposited on the fine tubeportions 21 and 22 as compared to the light emitting portion 20.Accordingly, the area, which is occupied by a ceramic such as alumina ofwhich the dielectric constant is higher than the dielectric constant ofiodide, is increased in the light emitting portion 20, so that astarting voltage is reduced.

As described above, in the discharge lamp 8, inert gas having negativepressure has been filled in the space that is formed outside the lightemitting tube 19 in the outer tube 18, a metal film 25 or a metal oxidefilm 25 has been applied or a metal wire 25 is wound on the outerperipheral surface of the fine tube portion 22 that is positioned on theside of the light emitting tube 19 corresponding to an anode, and anegative high-voltage pulse has been applied to the anode-side electrode24 at the time of the start-up of the discharge lamp.

Accordingly, it may be possible to reduce the starting voltage at thetime of the start-up of the discharge lamp, and to improve thereliability of the start-up of the discharge lamp and to reducemanufacturing costs of the discharge lamp by the reduction of thestarting voltage.

All the shapes and structures of the respective components described inthe exemplary embodiment of the invention are merely illustrative of anexemplary embodiment of the invention, and the scope of the inventionshould not be interpreted restrictively by the shapes and structures ofthe respective components.

1. A vehicular discharge lamp that emits light by a DC lighting system,the vehicular discharge lamp comprising: an outer tube that is mountedon a socket; a light emitting tube that is disposed in the outer tube,and that includes alight emitting portion and a pair of fine tubeportions connected to the light emitting portion on the opposite sidesof the light emitting portion; a cathode-side electrode and ananode-side electrode that are disposed in the light emitting tube; twolead wires which are respectively connected to the cathode-sideelectrode and the anode-side electrode, wherein at least apart of thecathode-side electrode and at least a part of the anode-side electrodeare disposed in the outer tube; and one of a coated layer or a metalwire, wherein the coated layer is one of a metal film or a metal oxide,wherein the coated layer is applied on an outer peripheral surface ofthe fine tube portion that is positioned on an anode side of the lightemitting tube, wherein the metal wire is wound on the outer peripheralsurface, wherein inert gas having negative pressure is filled in a spaceoutside the light emitting tube in the outer tube, and wherein anegative high-voltage pulse is applied to the anode-side electrode at atime of start-up.
 2. The vehicular discharge lamp according to claim 1,wherein tungsten to which thorium oxide is added is used as a materialto form the anode-side electrode.
 3. The vehicular discharge lampaccording to claim 1, wherein the maximum inner diameter of the lightemitting portion is set to 3 mm or less in a direction perpendicular toa direction where the pair of fine tube portions are connected to eachother.